A hydrogel has high biocompatibility because the main component of the hydrogel is water. In addition, it is a soft material with low environmental load. From these viewpoints, the hydrogel has attracted attention in recent years.
As a hydrogel having a highly strengthened self-supporting property, an organic-inorganic composite hydrogel obtained by carrying out polymerization reaction of a (meth)acrylamide derivative in the coexistence of a layered clay mineral homogeneously dispersed in water has been reported (Patent Document 1). As a similar report example, an organic-inorganic composite hydrogel having a polymer partially containing a carboxylic acid salt or a group having a carboxy anion structure in poly(meth)acrylamide and a clay mineral has been also known (Patent Document 2). In these report examples, the polymer to be formed and the clay mineral form a three-dimensional network structure by polymerizing monomers in the aqueous dispersion of the layered clay mineral and thus the organic-inorganic composite hydrogel is formed.
In these organic-inorganic composite hydrogels, however, unreacted monomers and reagents such as polymerization initiators that are concerned about toxicity may remain in the gel. For non-chemical manufacturers, the organic-inorganic composite hydrogel is difficult to produce and also difficult to mold a gel into any shapes because the hydrogel is formed after a chemical reaction.
As an organic-inorganic composite hydrogel having a self-supporting property, which can be produced by mixing at room temperature, the hydrogel containing a dendrimer compound having a polycationic functional group at its end and a layered clay mineral has been known (Patent Document 3). In this example, the dendrimer is produced by a multi-stage synthesis reaction and thus a problem of expensive production cost arises.
An organic-inorganic composite hydrogel that can be produced by simply stirring and mixing an electrolyte polymer, clay particles, and a dispersant has also been reported (Patent Document 4). In this organic-inorganic composite hydrogel, the clay particles homogeneously dispersed in the electrolyte polymer are crosslinked to form a gel structure. However, when the concentration of each component is increased in order to increase the strength of the gel, the viscosity of the hydrogel-formable composition is increased. In addition, the hydrogel-formable composition of which concentrations of each component are increased has a higher viscosity when the hydrogel-formable composition is mixed to form a gel. Consequently, for the organic-inorganic composite hydrogel described in Patent Document 4, production of the hydrogel may be remarkably difficult when the concentration of each component is increased in order to improve the strength of the gel.